HLA-G is a nonclassical MHC class I molecule which is expressed specifically in the placenta. To investigate the expression of MHC class I molecules in the rhesus monkey placenta, we cloned and sequenced all of the MHC class I cDNAs isolated from several different rhesus monkey placentae. Using HLA-G specific primers, we amplified, cloned and sequenced an unusual family of MHC class I cDNAs from rhesus trophoblasts, amniotic membrane, and fetal liver. These cDNAs all possess a stop codon at the beginning of exon 6, similar to the location of the premature stop codon of HLA-G. Gene trees constructed using nucleotide sequences of these novel cDNAs and MHC class I alleles from the rhesus monkey and other primates revealed that the rhesus placental cDNAs were related to the MHC class I A locus. Eight different cDNAs were isolated from seven samples, suggesting that this locus was polymorphic. We also determined that these cDNAs were alternatively spliced. cDNAs were isolated in which exon 2 was spliced in-frame to exon 4, and in which exon 2 was spliced in-frame to exon 5. These novel characteristics suggested that these cDNAs were encoded by a new locus, one which exhibits many of the characteristics of HLA-G, but is distinctly A-related. Thus, we have designated this locus Mamu-AG (Macaca mulatta-AG). In addition to Mamu-AG alleles, we have also identified two alleles of a rhesus MHC class I pseudogene which had significant sequence identity to HLA-G. These MHC class I cDNAs were isolated from rhesus monkey trophoblasts using RT/PCR. Each cDNA, however, contained numerous premature stop codons and frameshift mutations indicating that this locus was inactivated in the rhesus monkey. Gene trees constructed using human, gorilla, chimpanzee, and cotton-top tamarin MHC class I alleles revealed that the rhesus monkey pseudogene alleles clustered with G locus orthologs and not with orthologs from other MHC class I loci, indicating that we had isolated the rhesus monkey HLA-G ortholog. These data suggest that the rhesus monkey HLA-G ortholog has been inactivated and that an MHC class I A-locus may have evolved to subserve the function of HLA-G in the rhesus monkey. Key Words Rhesus, MHC